1. Field of the Invention
This invention relates generally to a coil assembly, and more particularly is directed to a coil assembly for a flat brushless motor.
2. Description of the Prior Art
In existing flat brushless motors, a rotor secured to the motor shaft is comprised of a rotor yoke or magnet casing and an annular permanent magnet, and a stator includes a plurality of coils which are angularly spaced apart about the axis of the shaft and disposed axially between the annular permanent magnet and an additional yoke which may be part of the rotor or stator. When the coils are energized in order, currents flowing therethrough interlink with magnetic flux from the permanent magnet so as to generate torque for rotating the motor shaft. In some conventional flat brushless motors, each of the coils is wound without a core or bobbin and is adhesively secured either to a printed circuit board, in the case where the additional yoke is part of the rotor, or to the additonal yoke in the case where the latter is part of the stator. In the latter case, an insulating material, such as insulating paper, is interposed between each coil and the additional yoke. In either case, the distance required between the permanent magnet and the additional yoke is increased by the thickness of the printed circuit board or of the insulating paper so that the magnetic resistance of the air gap is increased and results in reduced efficiency. Furthermore, it is dfficult to effect the accurate positioning of the coils on the additional yoke, and a special assembling device is needed therefor with the result that manufacturing costs are increased.
In other existing flat brushless motors, each coil is wound between flanges on a bobbin of synthetic resin, and each assembly of a coil and the respective bobbin is mounted on the stator yoke by way of one of the respective flanges. Although the flanged bobbin facilitates the accurate positioning of the respective coil on the stator yoke, the air gap is undesirably increased by the flange thickness. If the flanges are made thin to minimize the increase in the air gap, there is the danger that the flanges may be deformed due to heat generated by the respective coil and the strength of the attachment of the coil assembly to the stator yoke will be decreased.